Safety requirements specifications

This chapter describes the process of preparing safety requirements specification (SRS), the contents of which have been clearly described by the ISA and IEC standards. The chapter discusses the three development phases where the overall safety requirements are defined and then risk reduction tasks are allocated to a safety instrumented system (SIS) and non-SIS contributors. These development phases lead to the actual SRS for the safety system that is to be designed and installed. Some well-established methods for determining the safety integrity level (SIL) are described, the selection of which is a project choice dependent on company practices and the type of information available. The various component parts of the SRS leading to a checklist that could be used to provide a pro-forma specification document are also described. The normal practice in an operating or engineering company will be to develop a company-specific SRS template for use in all projects. The template will carry header sections defining the exact reference numbers and descriptions for the particular safety function. There is a complete document for each individual safety function although it is possible that the general section will be common to a set of applications. In this case, the general section lists each of the safety functions covered by the document.